Membrane or sheet liners are commonly used to line excavations or other containment facilities to prevent escapement therefrom of hazardous and/or polluting fluid wastes, solid waste leachates or valuable fluids. Such materials must be stored at the lowest possible cost on either a short term or a long term basis. Membrane liners consist of a number of membranes or flexible sheets of liner material joined together at their edges and joined to bounding structures to yield a continuous liner interposed between the fluid to be contained and the surroundings into which flow of the contained fluid is to be prevented.
Conventional membrane liners have a finite permeability and may suffer from a number of imperfections including holes in the membrane or sheet material which are inadvertently produced during manufacture of the material; holes which are inadvertently caused during the process of construction of the liner from the sheet or membrane material or during the process of installing the liner in the excavation or other containment region; imperfections in the welds or seams used to join adjacent segments of membrane or sheet material to form the liner; and, holes which develop in the liner after it is installed, due to punching, shearing, settling, chemical attack and a variety of other causes.
Liners are conventionally subjected to fluid pressures from both sides of the liner. Ambient air pressure subjects both sides of the liner to a first fluid pressure. Since this pressure is normally equal on both sides of the liner it does not produce a significant pressure gradient across the liner and therefore does not induce significant flow through the liner. Fluids contained in the region above the liner exert a second fluid pressure on the upper surface of the liner. Ground water in the region beneath the liner exerts a third fluid pressure on the lower surface of the liner. Accordingly, liners are conventionally subjected to fluid pressure gradients caused by the differential between the second and third fluid pressures. If the fluid pressure above the liner exceeds that beneath the liner, fluid will flow from the fluid containment region above the liner through any disruptions in the liner (i.e. holes, imperfect seams, etc.) and into the region beneath the liner, thus defeating the objective of the liner, which is to prevent such fluid escapement.
In practice, all liners have a finite permeability due to the inherent porosite of the material used to construct the liner. Accordingly, all liners leak at a small but finite rate. If holes are inadvertently made in the liner, or if the seams which join adjacent segments of liner material are imperfect, then the rate of leakage may increase dramatically and it is such increased leakage which is desirably prevented. In the prior art, high security composite liners have been constructed with double or even triple layers of liner material. Drainage layers are typically established between layers of liner material. Fluid which escapes through the uppermost liner passes into the drainage layer beneath the uppermost liner and flows towards drainage collection points established in the drainage layer. However, if holes occur in the liner located immediately beneath the drainage layer than secondary leakage may occur through the lower liner. If the leakage rate through the uppermost liner is sufficiently rapid then a localized fluid pressure may develop within the drainage layer and consequential high rates of leakage can occur through the lower liner before the fluid escaping through the uppermost liner can be collected and removed by the drainage system.
In practice, it is difficult to determine the rate of leakage through a field installed liner, particularly if the leakage rate is relatively low and particularly if a composite liner, comprised of multiple layers of liner material is involved. Large fluid losses may occur before the leakage is detected. If the stored fluid is valuable, or hazardous, or would produce a particularly undesirable impact on the surrounding environment, then such leakage should idealy be prevented. It is also desirable that the liner system be testable to determine whether the potential for leakage exists so that steps can be taken to prevent leakage.
The present invention provides a pressure barrier liner which significantly reduces the possibility of leakage through the liner, while facilitating testing of the liner to determine whether leakage could occur through the liner, even at relatively low leakage rates.